This article discusses four different flushing and cleaning methods for process lines and vessels during pre-commissioning.
- Cleaning with air
- Cleaning with steam
- Cleaning with water
- Mechanical cleaning.
Cleaning with air
During the cleaning operation, the air velocity should be as follows:
- In the gas process lines, the velocity should be the lowest figure between 60 m/s and the velocity obtained at 1.3 time the maximum process flow rate. The minimum velocity should be 30 m/s.
- In liquid process lines, a velocity between 30 m/s and 60 m/s should be used.
Two main methods can be used:
- Continuous blowing of air: Air is continuously blown from a source (directly from Air Compressor or via a large capacity vessel) at a high velocity through the lines (and vessels) to carry away the loose material. Due to the high velocity of the air, a light erosion of the walls generally occurs during the air blowing, which is beneficial with regards to cleaning.
- Quick decompression: A network is filled with compressed air up to the setting pressure of a rupture disk. When the rupture disk blows out, the source of air is stopped and the pressure inside the network decreases quickly. This operation will preferably be done using a "quick opening valve" after having pressurized the network to the desired level of pressure. The existing valves of the network should not be used for this purpose, but additional valves should be provided specially for these operations. The quick decompression causes a high velocity of air in the pipes, thus removing loose material and performing a light erosion on the walls which eliminate rust and scale.
Cleaning with Steam (for steam networks only)
Whenever steam is available, an excellent cleaning fluid is provided. Steam can be used, like air, for continuous blowing, and preferably, for intermittent blowing. The advantage of the latter is that between two blowings the pipes (and possibly vessels) cool and contract causing millscale and rust deposits to crack. These deposits are then easily removed in subsequent blowings. Steam blowing should be used only for steam networks.
Cleaning with water
Cleaning of piping by circulation of water: Line flushing uses water from the normal cooling or firewater systems. Water supply could conceivably be from other sources, i.e. City water lines, supply boat temporary pumps, etc., depending upon the ongoing situation. This type of flushing normally involves taking a section of piping and flushing it from a "startpoint" to an open end. A high velocity flow, in the order of 10-13 ft/s should be achieved in order to be effective. In some circumstances additional pumping capacity will be required. This method of cleaning is effective in removing loose debris from pipework but not particularly effective in removing rust and millscale from piping (or vessel) walls.
Fill and dump water flushing: Fill and dump flushing is often used for cleaning vessels. The method being quite simple, the adequately vented vessel is filled to the desired level with flush water and then dumped to grade, the flush water dislodges loose debris and rust and carries it out of the equipment. Note that some lines connected to the vessel can be flushed by dumping the water through them. One major problem with this method is the disposal of the large quantities of water involved which may overload the normal drain systems.
Cleaning of vessels or equipment by water spray or high pressure water: If there is only dust on the walls of a large vessel, a water spray from a hose pipe might be sufficient. High pressure water may be used to remove solid particles or foreign matter from inside walls or inside parts of vessel or equipment, in particular where access is a problem. Attention has to be given to the potential corrosiveness of water (chloride ions on stainless steel, seawater on carbon steel) and special water quality may be necessary (treated demineralized water or inhibited water).
Manual cleaning: When a system, or part of a system, can be entered, it may be possible to carry out pipewall or vessel wall cleaning manually using either rotary (i.e. air driven) or hand held steel wire brushes. Rust, scale, and dirt that are dislodged may be subsequently removed by the use of an industrial grade vacuum cleaner, by air blowing, or a combination of these two methods. Brushing should be avoided for vessels or piping fabricated from any grade of stainless steel. Carbon steel brushing may cause subsequent corrosion problems. In any event, stainless steel equipment is not likely to have significant deposits of surface dirt or oxide.
Line pigging: For equipment provided with pigging facilities, e.g. inter-platform pipelines, FPSO-platform pipelines, platform-to-shore pipelines, etc., special brush pigs are available. These are pigs equipped with peripheral wire spike type brushes and it is often the case that such pigs are used to dislodge rust and scale from pipeline walls. However, deposits of rust and scale are not necessarily conveyed from a pipeline by the use of such a projectile. Such deposits are normally removed by sending a "scraper" or cupped pig through the line which pushes these loose deposits of it for subsequent removal at an "open end".
Choice of the cleaning method should depend upon:
- The fluids available on site: compressed air, steam, seawater, fresh water, inhibited water and so on.
- The volume of the network: this determines the quantity of fluid to be used and the time needed to fill the network to the desired pressure (quick decompression method).
- The piping diameter
- Circulation of water is usually selected for diameters up to 6", because of the large flow required.
- Air blowing is acceptable for small diameter lines to be calculated according to the possible air flow and an average speed of 130 ft/s.
- Air quick decompression method can be used on any pipe diameter provided necessary precautions are taken.
- Mechanical cleaning should be used for vessels (manual cleaning) but also for large diameters (i.e. 30") pipes providing safety rules are respected (vessel entry).
- The design conditions of the system: some large diameter piping used for gas service cannot withstand their own weight when they are full of water. Some piping cannot withstand the expansion caused by steam blowing.
- The material specification: for stainless piping, air blowing is better (if the other conditions allow it) than water cleaning, to avoid the possibility of corrosion by chloride ions.
- The process function of the system:
- Water lines are usually flushed with water and inert gas and air lines with air – gas process lines should be air blown.
- Low operating temperature lines should be air blown.
- Hydraulic pipe work should be air blown.
Consider protecting the devices within its network while performing cleaning. During the flushing or the blowing of a line, some particles such as sand, rust, etc. are carried away at high velocities and may either damage some devices when an impact occurs, or clog small diameter pipes. Accordingly, fragile devices have to be removed from the pipe before flushing/blowing.
The following are considered as fragile devices:
- Control valves and check valves
- Safety valves
- Calibrated orifices
- Turbine type flow meters
- Pitot tube flow meters
- In-line flow totalizers
- Nozzles of ejectors
- Bellow valves
- Internals of filters and strainers
- Process impulse lines for instruments should be isolated by means of the block valve on the impulse pipe itself and, whenever possible, should be disconnected; this operation should be made by an instrument specialist
- Thermowells may remain installed if the pipe is not too dirty
- When flanges have to be opened to allow the flow to flush out, they should be checked that this operation does not create any stress on a pump, turbo-expander, compressor flange upstream or downstream
- The flushing is generally not carried out through columns or drums; but if it is necessary to go through such equipment, or if the vessel is used as an air reservoir, the removable internals have to be dismantled prior to starting the flushing, in order not to damage them
- Any column or drum to be used as an air reservoir must have its associated relief valve(s) in normal operation before pressurizing starts
Some devices are not in the near vicinity but close by also need to be protected from damages. During flushing and blowing operations, large quantities of dirty water and dirty air are spread out in the units. This water or air generally contains sand, particles of rust, etc., and might damage other equipment (for example: breaking the glass of a pressure gauge). Additionally, the water itself may damage the insulation materials, the electrical motors, etc., accordingly, the following actions should be taken where necessary:
- To put a tarpaulin on motors, insulation, instruments near the flow outlet
- To direct the outlet flow towards a place where there is no (fragile) equipment
- To change the direction of the flow by rotating an elbow
- To connect a hose pipe at the outlet to drive the flow to a desired location (for water flushing only)
- To add a dummy spool to extend the outlet further
- To install and fix properly, a sheet of metal (insulation cover sheet for example) perpendicularly to the flow, in order to break the flow impact
- On platforms, to protect the equipment and the people on the deck underneath
- Drains, which are not designed for such large flows, should not be used to evacuate water, as they might be flooded
Following are some cleaning steps preparations that should be considered. Whenever possible, the cleaning fluid should flow from top to bottom.
- The diameter of the outlet pipe should be at least two thirds of the diameter of the line to be cleaned, and at least as large as the inlet pipe.
- Main headers should be cleaned first (i.e. before their connecting lines).
- Large diameter pipes should be cleaned before small diameter ones.
- Dead ends should be avoided or, if not possible, cleaned separately.
- Fragile devices included in the network (all flanged valves, turbine-type flowmeters, orifice restrictions, etc.) should be removed.
- Protection of the devices in the vicinity of the network should be carefully studied, in particular near the water (or air) outlet, in order not to damage electric motors or instruments with water or dirty air.
- Existing block-valves on the lines to be cleaned should not be used to control an air flow as they could be damaged by erosion (due to the high velocity of the fluid).
- Special care should be taken for vessels designed only for an atmospheric pressure service, in order not to create, during washing, a high pressure due to a water head.
- Whenever a vessel full of water has to be emptied, a special note should be issued to state that a vent will have to be opened previously in order not to create a vacuum in the vessel.
- The cleaning fluid should not flow through a conventional exchanger unless the pipes upstream of the flushing flow have been cleaned (use the by-pass, if any, or "break" the line before the exchanger). It should never be flown through a plate exchanger.
- The cleaning fluid should never flow through pumps or compressors: the line should be disconnected before the equipment or a "jumper" should be installed.
- For large diameter pipes, some temporary supports, hangers, pads, etc, may have to be added to withstand the weight when a line is disconnected.
- During flushing, the spring supports, spring hangers, etc, should be blocked.
- When flanges have to be opened to allow the flow to flush out, the distance between the two flanges should be at least 1/6 of the pipe diameter.